This invention is related to a novel hot melt pressure sensitive adhesive composition for use with oriented polypropylene films. Specifically, the invention relates to a composition comprising a blend of SIS and SBS block copolymers combined with a blend of hydrocarbon resins which result in a low cost adhesive with properties similar to those made with SIS alone.
Packaging tape is one of the largest volume applications for pressure sensitive adhesives. While paper tapes and vinyl film tapes have been traditionally used in packaging, there has been increased interest in the use of oriented polypropylene films, particularly biaxially oriented polypropylene films.
Of the different adhesives used with oriented polypropylene films, block copolymer based hot melt adhesives are the most popular. These adhesives are generally based on styrene-isoprene block copolymers because of their superior performance over time. One draw back of such adhesive compositions is that they are relatively expensive when compared to adhesives based on other block copolymers such as styrene-butadiene block copolymers.
Styrene-butadiene block copolymer based adhesive compositions have been tried with oriented polypropylene tapes without success. While the initial adhesive properties are acceptable, the aged properties decline rapidly when compared to SIS based adhesive compositions. Furthermore, the required adhesion to paperboard was not achieved.
It would be useful; therefore, to develop an adhesive composition for use with oriented polypropylene tapes having the properties of a SIS based adhesive at a lower cost. The invention described herein accomplishes this by blending SIS block copolymers with SBS block copolymers. The hydrocarbon tackifier resin used must also consist of a blend of a SIS compatible resin and a SBS compatible resin. The resulting adhesive has the properties of an adhesive based on SIS alone but at a significantly lower cost.
U.S. Pat. No. 5,523,343 discloses a pressure sensitive hot melt adhesive composition comprising a blend of a radial SBS copolymer and a SIS block copolymer. The composition also includes a tackifier resin and plasticizer oils. The patent is silent concerning the use of a blended tackifier resin
The adhesive composition of the present invention comprises an elastomeric component and a tackifying resin component. The elastomeric component in turn comprises a blend of Styrene-Isoprene (SIS) block copolymers and Styrene-Butadiene (SBS) block copolymers. The SIS copolymer comprises from 20 to 80 wt % of the elastomeric component while the SBS copolymer comprises from 80 to 20 wt % of the elastomeric component. The preferred ranges of the SIS copolymer are from 30 to 80 wt %, more preferable 50 to 80 wt %. The preferred ranges of the SBS copolymer are from 70 to 20 wt %, more preferable 50 to 20 wt %.
The tackifier component comprises a blend of at least two hydrocarbon tackifier resins, the first comprising a resin compatible with SIS block copolymers and the second comprises a resin compatible with SBS copolymers. In the preferred embodiment, the relative ratios of the two resins should be similar to that for the block copolymers in the elastomeric component.
In the adhesive composition, the elastomeric component comprises from 40 to 60 wt %, with 45 to 55 wt % being preferred and 45 to 50 wt. % most preferred. The tackifying resin component comprises from 40 to 60 wt %, with 45 to 55 wt % preferred and 50 to 55 wt % most preferred. In the practice of the invention, the total amount of tackifier component in the final adhesive composition may exceed that of the elastomeric component. Thus, in practice, the tackifier component may exceed 100 wt. % based on the total elastomer component.
Typical performance characteristics achieved by the adhesive compositions of the present invention when applied to BOPP films at the coating weight of 20 grams/cm2 include (i) shear on cardboard at 40xc2x0 C., minimum 10 hours average and (ii) Ball tack, maximum 4 cm.
The invention relates to a novel hot melt pressure sensitive adhesive useful in the manufacture of packaging tapes manufactured with oriented polypropylene films. Specifically, the adhesive comprises an elastomeric component, which in turn comprises a blend of SIS and SBS block copolymers, and a tackifier component, which comprises a blend of a SIS compatible hydrocarbon resin and a SBS compatible hydrocarbon resin. The resulting adhesive exhibits the same attributes as adhesives produced from SIS alone at reduced manufacturing costs.
The elastomeric component comprises a blend of SIS and SBS block copolymers. The SIS block copolymer ranges from 20 to 80 wt %, preferably 30 to 80 wt % and more preferably 50 to 80 wt % of the Elastomeric Component and the SBS block copolymer comprises from 80 to 20 wt %, preferably 70 to 20 wt % and more preferably 50 to 20 wt % of the Elastomeric component.
The SIS block copolymer used in the Elastomeric Component of the adhesive is a thermoplastic elastomer having the structure (S-I)nxe2x88x921S wherein S is substantially a polystyrene block, I is substantially a polyisoprene block and n is an integer of from 2 to 10 wherein the content of the polystyrene in the thermoplastic elastomer ranges from 10 to about 30 wt % and wherein the number average molecular weight (Mn) of the thermoplastic elastomer ranges from about 50,000 to about 500,000. The preferred SIS block copolymer used in the invention is a triblock copolymer of the formula above wherein n=2, i.e., a linear polymer of the formula S-I-S wherein S is substantially a polystyrene block and I is substantially a polyisoprene block. These block copolymers may be prepared by well known anionic solution polymerization techniques using lithium-type initiators such as disclosed in U.S. Pat. Nos. 3,251,905 and 3,239,478, the complete disclosure of which is hereby incorporated by reference for purposes of United States practice.
Preferred block copolymers have a number average molecular weight (determined by GPC) in the range of from about 50,000 to 500,000, more preferably from about 90,000 to about 250,000, even more preferably 90,000 to 175,000 and most preferably 90,000 to 135,000. These block copolymers contain from about 10 to 30 wt % polymerized styrene, more preferably from about 15 to 25 wt % polymerized styrene and most preferably from about 16 to 20 wt % polymerized styrene.
The SIS block copolymer may also comprise a blend of two different SIS copolymers, one having a polystyrene block content of 10 to 20 wt % and the other having a different styrene content of from 15 to 35 wt %, blended to a ratio in the range of from 10:1 to 1:10 parts by weight. The use of two different SIS block copolymers offers the advantage of improved cohesive strength and more precisely tailoring polystyrene content in the preferred target range of from 15 to 25 wt %, or 16 to 20 wt % respectively.
Although pure triblock SIS copolymer is preferred (one having less than 0.1 wt % of diblock polymer) the SIS copolymer may also contain from about 0.1 to about 65 wt %, preferably less than or equal to 25 wt %, most preferably less than 20 wt % of the triblock copolymer of diblock copolymer having the structure S-I and containing from about 10 to 30 wt % polystyrene block. This material may be present as an impurity in the manufacture of the triblock copolymer or may be separately blended with the triblock as a further technique for achieving target polystyrene content or modifying the cohesive properties of the composition. Preferred number average molecular weights of the diblock SI copolymers range from about 100,000 to about 250,000.
While linear triblock SIS copolymers are preferred in the practice of this invention, radial SIS block copolymers may also be used. The radial SIS copolymers should have the same styrene levels as the linear copolymers discussed above, e.g., from about 10 to 30 wt % polymerized styrene. Radial SIS copolymers useful in the practice of the invention will have a molecular weight (Mn) of 180,000 to 250,000.
These linear and radial SIS block copolymers are available commercially and are prepared in accordance with methods known in the art. Examples of SIS copolymers useful in the practice of this invention include Vector(trademark) 4111, Vector(trademark) 4511 and Vector(trademark) 4113, manufactured by Dexco Polymers, LLP, Kraton(trademark) D 1107, Kraton(trademark) D 1160 and Kraton(trademark) D 1161 manufactured by Shell Chemical Company; Europrene(trademark) SOL T 190 and Europrene(trademark) SOL T 193 from Enichem: and Quintac(trademark) 3421, Quintac(trademark) 3422 and Quintac(trademark) 3433 from Nippon Zeon. An example of a radial block copolymer useful in the practice of this invention are Quintac(trademark) 3450 made by Nippon Zeon and Kraton(trademark) 1124 made by Shell Chemical and DPX 552 and 556, both available from Dexco Polymers, LLP.
Particularly preferred SIS block copolymers used in this invention have a melt flow rate in the range of from about 5 to 40 g/10 min., as measured by ASTM D 1238-82 using condition G (200xc2x0 C., 5 kg. weight).
The SBS block copolymer used in the Elastomeric Component of the adhesive is a thermoplastic elastomer having the structure S-B or (S-B)nxe2x88x921S wherein S is substantially a polystyrene block, B is substantially a polybutadiene block and n is an integer of from 2 to 10 wherein the content of the polystyrene in the thermoplastic elastomer ranges from 10 to about 40 wt % and wherein the number average molecular weight (Mn) of the thermoplastic elastomer ranges from about 50,000 to about 500,000. The preferred SBS block copolymer used in the invention is a triblock copolymer of the formula above wherein n=2, i.e., a linear polymer of the formula S-B-S wherein S is substantially a polystyrene block and is substantially a polybutadiene block. These block copolymers may be prepared by well known anionic solution polymerization techniques using lithium-type initiators such as disclosed in U.S. Pat. Nos. 3,251,905 and 3,239,478, the complete disclosure of which is hereby incorporated by reference for purposes of United States practice. Radial SBS copolymers may also be used in the practice of the invention.
Preferred block copolymers have a number average molecular weight (determined by GPC) in the range of from about 50,000 to 500,000, more preferably from about 100,000 to about 180,000, even more preferably 110,000 to 160,000 and most preferably 110,000 to 140,000.
Although pure triblock SBS copolymer is preferred (one having less than 0.1 wt % of diblock polymer) the SBS triblock copolymer may also contain from about 0.1 to about 35 wt %, preferably less than or equal to 25 wt %, most preferably less than 20 wt % of the triblock copolymer of diblock copolymer having the structure S-B and containing from about 10 to 30 wt % polystyrene block. This material may be present as an impurity in the manufacture of the triblock copolymer or may be separately blended with the triblock as a further technique for achieving target polystyrene content or modifying the cohesive properties of the composition. Preferred number average molecular weights of the diblock SB copolymers range from about 100,000 to about 250,000.
These linear and radial SBS block copolymers are available commercially and are prepared in accordance with methods known in the art. Examples of SBS copolymers useful in the practice of this invention include Vector 8505(trademark), Dexco Polymers, LLP, Kraton(trademark) D 1102, manufactured by Shell Chemical Company; Europrene(trademark) SOL T 166 manufactured by Enichem and Finaprene(trademark) 411 manufactured by Fina. Examples of radial SBS copolymers useful in the practice of the invention include Kraton(trademark) D 4141 and Kraton(trademark) D 4158.
As discussed above, the Tackifier component may comprises a blend of at least two hydrocarbon-tackifying resins. The first is a resin compatible with the SIS copolymer and the second is a resin compatible with the SBS copolymer. The SIS compatible resin is present in an amount ranging from 20 wt % to 80 wt %, preferably 30 to 80 wt %, more preferably 50 to 80 wt % based on the entire tackifier component. Similarly, the SBS compatible resin is present in an amount ranging from 80 to 20 wt %, preferably 70 to 20 wt %, more preferably 50 to 20 wt %. In the preferred embodiment, the amount of resin present will be proportional to the amount of the compatible copolymer present. For example if the SIS copolymer comprises 20 wt % of the Elastomeric Component, then the SIS compatible resin will comprise 20 wt % of the Tackifier composition. Similarly, if the SBS copolymer comprises 80 wt % of the Elastomeric Component, then the SBS compatible resin will comprise 80 wt % of the Tackifier component.
Other ratios, however, are possible within the scope of the invention. For example, the SBS compatible resin can be higher in proportion than the SBS block copolymer, i.e., at 30% SBS, the SBS compatible resin could be also 50% of the resin composition. Vice versa at 50% SBS, the SBS compatible resin could also be 30%. The most preferred ratio is the one mentioned in the application, but we need the flexibility for ratios as mentioned above.
The SIS compatible resin used in this invention is preferably a petroleum resin prepared by the polymerization of a petroleum cracked distillate generally boiling in the range of 25xc2x0 C. to 80xc2x0 C. and a monovinyl aromatic monomer having 8 or 9 carbon atoms in the proportions to yield a resin containing 20 wt % or less of the monovinyl aromatic compound as determined by Nuclear Magnetic Resonance analysis. The monovinyl aromatic content may range from 5 to 20 wt % and desirably from 5 to 15 wt %. The petroleum-cracked distillate comprises a mixture of saturated and unsaturated monomers, the unsaturated monomers being monoolefins and diolefins and although the unsaturated materials are predominantly C5, some higher and lower material such as C6 olefins and diolefins may be present. The distillate may also contain saturated or aromatic materials, which can act as a polymerization solvent.
The SIS compatible tackifiers will generally have a ring and ball softening point of from 50 to 140xc2x0 C., preferably 85 to 105 and most preferred, 90 to 100. The number average molecular weight (Mn) will range from 700 to 1500, with 800 to 1200 preferred. The weight average molecular weight (Mw) shall range from 1200 to 2000, with 1300 to 1700 preferred. The molecular weight distribution (Mw/Mn) will range from 1.3 to 2.5 with 1.8 preferred. The Z average value (Mz) may range from 2500 to 5000 with 3000 to 4000 preferred.
The preferred monovinyl aromatic monomer is styrene, which may be substituted, in the aromatic group. Alternatively, alpha-methyl styrene or vinyl toluene may be used. Commercially available mixtures of vinyl aromatic monomers may be used as well.
The resins are conveniently prepared by Friedel-Crafts catalyzed polymerization in which the mixture of cracked distillate and monoaromatic monomer are treated with 0.25-2.5 wt. % of a catalyst such as aluminum chloride, aluminum boride, or solutions, slurried or complexes thereof or borontrifloride. The polymerization mixture may also include from about 10 to 100 wt % of a chain transfer agent such as diisobutene oligomer to obtain resins having a narrow molecular weight distribution. These reactions are generally carried out at temperatures in the range of 0xc2x0 to 120xc2x0, preferably 0xc2x0 to 80xc2x0, more preferably 20xc2x0 to 55xc2x0 C., the conditions being controlled to yield a resin of the required softening point. Residual catalyst is quenched by suitable methods such as the addition of methyl alcohol and subsequent filtration followed by water and/or caustic washing. The final solution may then be stripped of unreacted hydrocarbons and low molecular weight oils by vacuum or steam distillation.
The most preferred SIS tackifier resins have ring and ball softening points in the range of from about 85xc2x0 to about 105xc2x0 C., more preferably in the range of from about 90xc2x0 to about 100xc2x0 C. and a monovinyl aromatic monomer (e.g., styrene) content in a range from 5 to 15 wt %, or in a range from 6 to 12 wt %, and preferably less than 10 wt. %.
A particularly preferred SIS compatible tackifier for use in the present invention has the following typical properties:
These resins and their method of manufacture are more generally disclosed in U.S. Pat. No. 4,078,132, the complete disclosure of which is hereby incorporated by reference for the purpose of U.S. patent practice.
Alternatively, the SIS compatible resin may comprise a petroleum resin prepared in a manner similar to that described above, but where the monovinyl aromatic monomer is generally absent. These resins are prepared from C5 monoolefins and diolefins and are generally referred to as aliphatic resins. Typical aliphatic resins have a ring and ball softening point of from 80 to 115xc2x0 C.; a number average molecular weight of from 600 to 1200 and an aromaticity level of less than 4 wt. %.
Examples of commercially available SIS compatible tackifier resins include ESCOREZ 2203, ESCOREZ 1310, ECR 411 AND ESCOREZ 2596, manufactured by Exxon Chemical Company; Piccotac 212 and Hercotac 1148, manufactured by Hercules. Inc.; Quintone D and Quintone U 185 manufactured by Nippon Zeon; Marukares R100 manufactured by Maruzen; and Wingtack Extra and Wingtack Plus manufactured by Goodyear Tire and Rubber Company.
The SBS compatible resin used in this invention is preferably a petroleum resin prepared by the polymerization of a petroleum cracked distillate generally boiling in the range of 25xc2x0 C. to 80xc2x0 C. and an olefinically unsaturated aromatic monomer having 8 or 10 carbon atoms in the proportions to yield a resin containing from 20 to 45 wt % of the olefinically unsaturated aromatic compound as determined by Nuclear Magnetic Resonance analysis. The petroleum-cracked distillate comprises a mixture of saturated and unsaturated monomers, the unsaturated monomers being monoolefins and diolefins and although the unsaturated materials are predominantly C5, some higher and lower material such as C6 olefins and diolefins may be present. The distillate may also contain saturated or aromatic materials, which can act as a polymerization solvent.
The preferred olefinically unsaturated aromatic monomer is styrene, which may be substituted, in the aromatic group. Alternatively -methyl styrene, vinyl toluene, indene and mixtures may be used.
One source for the olefinically unsaturated compound is heart cut distillate (HCD) obtained by fractionation. The typical composition of HCD is as follows:
The resins are conveniently prepared by Friedel-Crafts catalyzed polymerization in which the mixture of cracked distillate and olefinically unsaturated aromatic monomer are treated with 0.1-3 wt. %, preferably 0.5 to 1.5 wt %, of a catalyst such as aluminum chloride, aluminum boride, or solutions, slurried or complexes thereof or borontrifloride. The polymerization mixture may also include from about 10 to 100 wt % of a chain transfer agent such as diisobutene oligomer to obtain resins having a narrow molecular weight distribution. These reactions are generally carried out at temperatures in the range of xe2x88x9220xc2x0 to 120xc2x0, preferably 30xc2x0 to 80xc2x0, the conditions being controlled to yield a resin of the required softening point. Residual catalyst is quenched by suitable methods such as the addition of methyl alcohol and subsequent filtration followed by water and/or caustic washing. The final solution may then be stripped of unreacted hydrocarbons and low molecular weight oils by vacuum or steam distillation.
The most preferred SBS tackifier resins have ring and ball softening points in the range of from about 50xc2x0 to about 100xc2x0 C., more preferably in the range of from about 70xc2x0 to about 95xc2x0 C and a olefinically unsaturated aromatic monomer (e.g., styrene) content of from 20 to 45 wt %, preferably greater than 22 wt %. Other suitable tackifier resins include rosins and esterified rosins, such as those described in U.S. Pat. No. 5,820,749, the complete disclosure of which is hereby incorporated by reference for purposes of United States practice.
A particularly preferred SBS compatible tackifier for use in the present invention has the following typical properties:
These resins and their method of manufacture are more generally disclosed in U.S. Pat. No. 4,078,132, the complete disclosure of which is hereby incorporated by reference for the purpose of U.S. patent practice.
Examples of commercially available SBS compatible tackifier resins include ESCOREZ 2101, ESCOREZ 5690, and ECR 373, manufactured by Exxon Chemical Company; Regalrez 5095 and Regalrez 3102 manufactured by Hercules. Inc.; Quintone U 190 manufactured by Nippon Zeon; and Wingtack 86 manufactured by Goodyear Tire and Rubber Company, Sylvalite RE 885 and Sylvatac RE 85 available from Arizona Chemical and Staybelite Ester 3 and Pentalyn H available from Hercules.
In the case of both the SIS compatible resins and the SBS compatible resins, the resins may be hydrogenated to reduce the levels of residual unsaturation. Hydrogenation results in reducing the color of the resins. Methods for hydrogenating petroleum resins are well known in the art and include the methods described in U.S. Pat. Nos. 4,629,766, 3,926,878 and 5,820,749. As with the non-hydrogenated resins, the styrene content and MW of the resin appears to be the key to compatibility with the SIS or SBS copolymers.
The adhesive compositions of the invention may also include other additives known in the art such as hydrocarbon extender oils, antioxidants, colorants, fillers and the like.
Suitable extender oils include aromatic, naphthenic or paraffinic oils and mixtures thereof. When used, the extender oil is added at a level from about 0.5 to about 25 wt percent of the elastomeric component, more preferably from about 5 to 15 wt %.
Suitable antioxidants include hindered phenols such as 2,6-di-t-butyl4-methylphenol; 1,3,5-trimethyl-2,4,6-tris (3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxybenzyl)-benzene; tetrakis[(methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)] methane (IRGANOX(trademark) 1010); octadecyll-3,5-di-t-butyl-4-hydroxy cinnamate (IRGANOX(trademark) 1076); and like known materials. Where present, the antioxidant is used at a preferred level of from 0.05 to about 2.0 wt % based on the elastomeric component.
The components of the adhesive composition may be blended by mixing them using any suitable mixing device at a temperature above the melting point of the components, e.g. at 130 to 180xc2x0 C. for a period of time sufficient to form a homogeneous mixture, normally 1 to 120 minutes depending on the type of mixing device.
In the case of continuous mixing such as practiced by most commercial manufacturers, a twin screw extruder is used to mix the adhesive components. First the elastomers are introduced into the extruder and mixed until the polymers have melted and are well mixed. Then the tackifiers are added followed by any oils which might be desired. To the extent pigments and antioxidants are used, they are normally blended in with the elastomers. The total mixing time is typically 1.5 to 2.0 minutes.
In the case of batch mixing, such as in the examples below, both polymers are added along with 20% of the tackifier component. When the polymers and tackifier reaches a homogeneous state, the remaining tackifier component is gradually added to the mix. Once all the tackifier component has been added and homogeneous mix is achieved, the balance of the plasticizer oil, antioxidants and any pigments are added. The total mixing time may run for up to 120 minutes.
The adhesive composition of this invention may be applied to a substrate as a melt then cooled using conventional procedures. The substrate in the present case is an oriented polypropylene film prepared using conventional techniques known in the art. Examples of suitable film include biaxially oriented polypropylene. The film thickness may vary from 12.5 to 112 xcexcm and desirably from 10 to 110 xcexcm depending on the film use and the proposed end use. For example, packaging tapes made with biaxially oriented polypropylene will often have a film thickness from 25 to 35 xcexcm whereas strapping tapes made with mono-oriented polypropylene films will range from 40 to 112 xcexcm.
The adhesive composition is applied to the substrate using conventional coating techniques such as roller coaters, die coaters and blade coaters, generally at a temperature of from 150xc2x0 to 200xc2x0 C. For example, the ready mixed Hot Melt Pressure Sensitive Adhesive (HMPSA) heated to the temperature where it can flow readily, usually temperatures from 170xc2x0 to 190xc2x0 C., can be applied to oriented polypropylene by any known techniques, preferably using a slot die.
A slot die is a closed system where the HMPSA is pumped through by a positive displacement pump. The slot die usually includes a rotating bar at the point of the outlet of the HMPSA in order to maintain a smooth surface.
Due to required thin channels inside the die through which the HMSPA has to flow, it is obvious that there are melt viscosity limits for the HMPSA depending upon required throughput. Thus one can say that, the higher the coating speed is, the lower the melt viscosity needs to be to obtain consistent and even coating at the desired thickness on the film.
The substrate should be coated with sufficient adhesive composition to provide a dry coat weight of from 10 to about 40 g/cm2. Generally, in the manufacture of tapes using oriented polypropylene, a dry coat weight of from about 10 to 30 g/cm2 is used.
After coating, the coated film is slit to the required dimension. In the manufacture of tape, the film is slit into strips and rolled into a finished product. The film may also be cut into shaped items to provide labels or medicinal tapers.